Multicore fiber

Abstract

This multicore fiber (1) comprises multiple cores (10) and a cladding (20), wherein an outer circumferential surface of the cladding (20) has a first outer circumferential surface (21) forming one circumferential portion thereof and a second outer circumferential surface (22) forming another circumferential portion thereof, and the first outer circumferential surface (21) bulges outward with a radius of curvature larger than that of the second outer circumferential surface (22).

Description

Technical Field

[0001] The present invention relates to a multi-core fiber.

Background Art

[0002] In recent years, with the spread of optical fiber communication systems, the amount of information transmitted by optical fibers has increased dramatically. Against this background, multi-core fibers in which the outer peripheries of a plurality of cores are surrounded by a single cladding are being used. Since a multi-core fiber can transmit a plurality of signals by the light propagating through each of the plurality of cores, the transmission capacity per optical fiber is increased.

[0003] When a multi-core fiber is used for long-distance optical signal transmission, it may be connected to another multi-core fiber. In this case, from the viewpoint of reducing the optical loss at the connection portion of the multi-core fiber, it is desirable to improve the arrangement accuracy of each core of the multi-core fiber.

[0004] Patent Document 1 below describes a multi-core fiber in which the outer shape of the cladding is non-circular in order to achieve good optical coupling. Patent Document 1 describes, for example, a multi-core fiber having a so-called D-shaped outer cladding in which a part of the outer peripheral surface is formed in a flat shape.

[0005]

Patent Document 1

Summary of the Invention

[0006] When aligning a D-shaped cladding multicore fiber in the rotational direction, alignment may be performed using side-view alignment, which involves irradiating the cladding from the side and observing the position of the cores. However, when performing side-view alignment on a D-shaped cladding multicore fiber, there are rotational positions where spherical aberration is large and side-view alignment is difficult. Such rotational positions include those where light is incident from a planar portion of the outer surface of the cladding. For this reason, alignment can be difficult.

[0007] Therefore, the present invention aims to provide a multi-core fiber that can be easily aligned.

[0008] One aspect of the present invention for solving the above problems is a multicore fiber comprising a plurality of cores and a cladding surrounding the cores, wherein the outer circumferential surface of the cladding has a first outer circumferential surface which is a part in the circumferential direction and a second outer circumferential surface which is another part in the circumferential direction, and at least a part of the first outer circumferential surface bulges outward from the cladding with a larger radius of curvature than the second outer circumferential surface.

[0009] When performing side-view alignment on such a multicore fiber, even when light is incident from the first outer surface, spherical aberration can be suppressed compared to when light is incident from a planar portion of the outer surface of the D-type cladding. Therefore, alignment can be easily achieved with the multicore fiber of the present invention.

[0010] Furthermore, embodiment 2 of the present invention is a multicore fiber of embodiment 1, characterized in that a plurality of cores are arranged on the outer periphery of the cladding, and at least one of the portions of the outer periphery facing the plurality of cores is the first outer periphery.

[0011] The portion of the outer surface facing the core is the portion of the outer surface closest to the core. Therefore, the distance between the core located on the outer periphery and the first outer surface facing that core is smaller than the distance between the core and the second outer surface when the portion of the outer surface facing the core located on the outer periphery is the second outer surface. Consequently, unwanted higher-order mode light propagating through the core facing the first outer surface can be attenuated by the influence of the first outer surface. Unwanted higher-order mode light is, for example, light not used for communication.

[0012] Furthermore, embodiment 3 of the present invention is a multicore fiber of embodiment 2, characterized in that each of the aforementioned parts is the first outer peripheral surface.

[0013] In this case, unwanted higher-order modes of light propagating through each core located on the outer periphery can be attenuated by the influence of the first outer surface.

[0014] Furthermore, embodiment 4 of the present invention is a multicore fiber according to any of embodiments 1 to 3, characterized in that the outer surface of the cladding has a non-rotationally symmetric shape.

[0015] In this case, the outer shape of the cladding can be used to coarsely center the multicore fibers so that the rotation direction of the cladding is in a specific direction.

[0016] Furthermore, aspect 5 of the present invention is a multicore fiber according to any one of aspects 1 to 4, characterized in that the radius of curvature of the first outer surface is 1.5 times or more and 20 times or less the radius of curvature of the second outer surface.

[0017] The ratio of the radius of curvature of the first outer surface to the radius of curvature of the second outer surface has this relationship, which makes it easier to perform coarse centering based on the outer surface of the cladding and suppresses spherical aberration when performing side-view centering.

[0018] As described above, the present invention provides a multicore fiber that can be easily aligned. [Brief explanation of the drawing]

[0019] [Figure 1] This figure shows a cross-section perpendicular to the longitudinal direction of a multicore fiber according to the first embodiment of the present invention. [Figure 2] This figure shows the spherical aberration when light is incident on the second outer surface of the multicore fiber shown in Figure 1 from a direction perpendicular to the longitudinal direction, and the light is emitted from the first outer surface. [Figure 3] This figure shows the spherical aberration when light is incident on the first outer surface of the multicore fiber from a direction perpendicular to the longitudinal direction and emitted from the second outer surface. [Figure 4] This figure shows a cross-section perpendicular to the longitudinal direction of a multicore fiber according to a second embodiment of the present invention. [Figure 5] This figure shows a cross-section perpendicular to the longitudinal direction of a multicore fiber according to a third embodiment of the present invention. [Modes for carrying out the invention]

[0020] Hereinafter, preferred embodiments of the multicore fiber according to the present invention will be described in detail with reference to the drawings. The embodiments illustrated below are provided to facilitate understanding of the present invention and are not intended to limit its interpretation. The present invention can be modified and improved from the embodiments within the scope of the claims without departing from its spirit. Note that for ease of understanding, the scale of each figure may differ from the scale described below.

[0021] (First Embodiment) Figure 1 shows a cross-section perpendicular to the longitudinal direction of a multicore fiber according to this embodiment. The multicore fiber 1 of this embodiment comprises a plurality of cores 10, markers 15, a cladding 20 that completely surrounds the outer surfaces of each core 10 and marker 15, an inner coating layer 31 that covers the outer surface of the cladding 20, and an outer coating layer 32 that covers the outer surface of the inner coating layer 31. In the example in Figure 1, an example with four cores 10 is shown.

[0022] In the multi-core fiber 1 of the present embodiment, each core 10 is arranged on the circumference centered on the reference position 20R which is approximately the center of the cladding 20. In the present embodiment, each core 10 is arranged on the outermost peripheral side of the cladding 20. In the multi-core fiber 1 of the present embodiment, the distances between the respective cores 10 are equal to each other, and each core 10 is arranged at a position that is approximately rotationally symmetric about 4 times around the reference position 20R. The diameter of the core 10 is, for example, 4 μm or more and 14 μm or less.

[0023] The markers 15 are arranged outside the circumference where each core 10 is arranged. The refractive index of the marker 15 may be higher or lower than that of the cladding 20 as long as it is different from the refractive index of the cladding 20.

[0024] The refractive index of each core 10 is higher than the refractive index of the cladding 20, and the relative refractive index difference of each core 10 with respect to the cladding 20 is, for example, 0.2% or more and 2.0% or less. Such a core 10 is made of, for example, silica glass to which a dopant such as germanium that increases the refractive index is added, and the cladding 20 is made of, for example, silica glass to which no dopant is added. Also, the core 10 may be made of silica glass to which no dopant is added, and the cladding 20 may be made of silica glass to which a dopant such as fluorine that decreases the refractive index is added. The marker 15 is composed of silica glass having a refractive index different from that of the cladding 20.

[0025] The cladding 20 has a non-circular outer shape and includes a first outer surface 21, which is a portion in the circumferential direction, and a second outer surface 22, which is another portion in the circumferential direction. In this embodiment, one portion of the outer surface of the cladding 20 facing each core 10 is designated as the first outer surface 21, and the other portion of the outer surface of the cladding 20 is designated as the second outer surface 22. The portion of the outer surface facing the core 10 is the portion of the outer surface closest to the core 10. Therefore, the distance between the core 10 facing the first outer surface 21 and the first outer surface 21 is smaller than the distance between the core 10 and the second outer surface 22. The second outer surface 22 overlaps with a portion of a predetermined circumference 20C centered on the reference position 20R of the cladding 20. For the sake of readability of the drawing, the circumference 20C shown by the dashed line and the second outer surface 22 are depicted with a slight offset. The first outer surface 21 bulges outward from the cladding 20 with a larger radius of curvature than the second outer surface 22 and is connected to the second outer surface 22. Therefore, the first outer surface 21 is located inside the circumference 20C.

[0026] In this embodiment, the length of the straight line connecting both ends of the first outer surface 21 is greater than or equal to the diameter of the core 10 facing the first outer surface 21. Also, in this embodiment, when viewing the first outer surface 21 along a direction perpendicular to the longitudinal direction of the multicore fiber 1, the core 10 facing the first outer surface 21 completely overlaps the first outer surface 21.

[0027] As described above, in this embodiment, the cladding 20 has a first outer surface 21 formed at only one location, so the outer surface of the cladding 20 has a non-rotationally symmetric shape.

[0028] Preferably, the radius of curvature of the first outer surface 21 is 1.5 times or more and 20 times or less the radius of curvature of the second outer surface 22. By having the radius of curvature of the first outer surface be 1.5 times or more the radius of curvature of the second outer surface, the outer surface of the cladding can become even closer to a planar shape. Therefore, rough centering based on such an outer surface of the cladding 20 can be performed more easily. In addition, by having the radius of curvature of the first outer surface be 20 times or less the radius of curvature of the second outer surface, spherical aberration can be suppressed when performing side-view centering.

[0029] The inner coating layer 31 and the outer coating layer 32 are each made of a resin such as an ultraviolet-curable resin, and the inner coating layer 31 and the outer coating layer 32 are made of different resins.

[0030] To manufacture a multicore fiber 1 having a cladding 20 of this shape, for example, a portion of the outer surface of the cladding rod, which will become the cladding 20, is cut from the base material of the multicore fiber 1. By drawing the base material having such a cladding rod to form a multicore fiber 1, the cut portion of the outer surface of the cladding rod becomes the first outer surface 21, and the other portion becomes the second outer surface 22.

[0031] Figure 2 shows the spherical aberration when light is incident on the second outer surface 22 of the multicore fiber 1 from a direction perpendicular to the longitudinal direction and emitted from the first outer surface 21. Note that when light is transmitted through the multicore fiber 1 in this way, the inner coating layer 31 and the outer coating layer 32 are stripped away. In Figure 2, since the first outer surface 21 is located on the light emission side, the multicore fiber 1 can be understood as a biconvex lens close to a planar lens, and the light emitted from the first outer surface 21 has a spherical aberration of magnitude a1. A planar lens is a lens in which the light incidence surface is convex and the light emission surface is planar, while a biconvex lens is a lens in which both the light incidence surface and the light emission surface are convex. Figure 2 also shows the magnitude a0 of the spherical aberration when the same light is transmitted through a multicore fiber with a circular cladding. A multicore fiber with circular cladding can be understood as a biconvex lens. Thus, by transmitting light through the multicore fiber 1 of this embodiment as described above, the spherical aberration of the light can be reduced compared to when light is transmitted through a multicore fiber having a circular cladding whose radius is smaller than the spherical aberration of the light when light is incident on the circular outer surface with a radius equal to the radius of curvature of the second outer surface from a direction perpendicular to the longitudinal direction of the circular cladding and the light is emitted from the circular outer surface. This reduced spherical aberration of the light allows for more precise alignment of the rotational direction of the multicore fiber 1.

[0032] Furthermore, Figure 2 shows the magnitude of spherical aberration aD1 when light is transmitted in the same manner to a multicore fiber having a so-called D-type cladding, where the portion corresponding to the first outer surface 21 of the cladding is planar. In Figure 2, this plane is indicated by a dotted line. In this case, the multicore fiber can be understood as a planar lens, and the magnitude of spherical aberration aD1 is smaller than the magnitude of spherical aberration a1 when light is transmitted through the multicore fiber 1 of this embodiment as described above.

[0033] Figure 3 shows the spherical aberration when light is incident on the first outer surface 21 of the multicore fiber 1 from a direction perpendicular to the longitudinal direction and the light is emitted from the second outer surface 22. As shown in Figure 3, in this case, the multicore fiber 1 can be understood as a biconvex lens close to a plano-convex lens, and the light emitted from the second outer surface 22 has a spherical aberration of magnitude a2. A plano-convex lens is a lens in which the surface on which light is incident is formed to be flat and the surface on which light is emitted is formed to be convex. The magnitude a2 of the spherical aberration is larger than the magnitude a1 of the spherical aberration described above. Also in Figure 3, the magnitude a0 of the spherical aberration when the same light is transmitted through a multicore fiber having a circular cladding is shown. In this case, the magnitude a2 of the spherical aberration tends to be larger than the magnitude a0 of the spherical aberration.

[0034] Furthermore, Figure 3 shows the magnitude of spherical aberration aD2 when light is transmitted in the same manner to a multicore fiber having a so-called D-type cladding, where the portion corresponding to the first outer surface 21 of the cladding is planar. In Figure 3, this plane is shown by a dotted line. In this case, the multicore fiber can be understood as a plano-convex lens, and the magnitude of spherical aberration aD2 is larger than the magnitude of spherical aberration a2 when light is transmitted through the multicore fiber 1 of this embodiment as described above.

[0035] As explained in Figures 2 and 3, when light is transmitted through the multicore fiber 1 of this embodiment from a direction perpendicular to the longitudinal direction and the multicore fiber 1 is rotated, the amount of change in spherical aberration can be suppressed more effectively than with a multicore fiber having a D-type cladding.

[0036] Furthermore, as long as the radius of curvature of the first outer surface 21 is greater than the radius of curvature of the second outer surface 22, the first outer surface 21 may have a constant radius of curvature that coincides with a part of the circumference of a hypothetical circle (not shown), or it may not have a constant radius of curvature. For example, if the radius of curvature of the first outer surface 21 is greater than the radius of curvature of the second outer surface 22, the shape of the first outer surface 21 may be part of an ellipse or part of a perfect circle.

[0037] As described above, the multicore fiber 1 of this embodiment has a cladding 20 outer surface that is a first outer surface 21, which is a part in the circumferential direction, and a second outer surface 22, which is another part in the circumferential direction. The first outer surface 21 bulges outward from the cladding 20 with a larger radius of curvature than the second outer surface 22. Therefore, when performing side-view alignment, even when light is incident from the first outer surface 21 side, spherical aberration can be suppressed compared to when light is incident from a planar portion of the outer surface of a D-type cladding, and the amount of change in spherical aberration can be suppressed when the multicore fiber 1 is rotated compared to a multicore fiber with a D-type cladding. Therefore, alignment can be easily performed with the multicore fiber 1 of this embodiment. In addition, since the shape of the outer surface of the cladding 20 is non-circular, rough alignment can be performed based on the outer shape of the cladding 20 before performing side-view alignment.

[0038] Furthermore, if the outer surface of the cladding 20 is non-rotationally symmetrical, as in the multicore fiber 1 of this embodiment, the alignment position can be fixed to a single position when performing the coarse alignment described above, making alignment easier.

[0039] Furthermore, in the multicore fiber 1 of this embodiment, the first outer surface 21 is the portion of the outer surface of the cladding 20 that faces the core 10 located on the outer periphery. Therefore, the distance between the core 10 facing the first outer surface 21 and the first outer surface 21 is smaller than the distance between the core 10 facing the second outer surface 22 and the second outer surface 22. Here, the closer the distance between the outer surface of the core 10 and the outer surface of the cladding 20, the easier it is for higher-order modes propagating through the core 10 to propagate to the cladding 20. Therefore, light of higher-order modes that are unnecessary for communication, etc., propagating through the core 10 facing the first outer surface 21 can be attenuated by the influence of the first outer surface 21.

[0040] (Second Embodiment) Next, a second embodiment of the present invention will be described in detail with reference to Figure 4. Note that components identical or equivalent to those in the first embodiment are denoted by the same reference numerals unless otherwise specified, and redundant descriptions are omitted.

[0041] As shown in Figure 4, the multicore fiber 1 of this embodiment differs from the multicore fiber 1 of the first embodiment in that each portion of the outer surface of the cladding 20 facing the core 10 is designated as a first outer surface 21, and the other portions of the outer surface of the cladding 20 are designated as a second outer surface 22.

[0042] In the multicore fiber 1 of this embodiment, each portion of the outer surface of the cladding 20 facing the core 10 is designated as the first outer surface 21. Therefore, compared to a multicore fiber having a circular cladding that overlaps with the second outer surface 22, unwanted higher-order mode light propagating through each core 10 can be attenuated by the influence of the first outer surface 21.

[0043] (Third embodiment) Next, a third embodiment of the present invention will be described in detail with reference to Figure 5. Note that components identical or equivalent to those in the first embodiment are denoted by the same reference numerals unless otherwise specified, and redundant descriptions are omitted.

[0044] As shown in Figure 5, the multicore fiber 1 of this embodiment differs from the multicore fiber 1 of the first embodiment in that the multiple cores 10 are arranged in a straight line.

[0045] In this embodiment, the cores 10 located at both ends are cores positioned on the outer periphery of the cladding 20. In this embodiment, each portion of the outer periphery of the cladding 20 facing the cores 10 at both ends is designated as the first outer periphery 21, and the other portions of the outer periphery of the cladding 20 are designated as the second outer periphery 22. The shape of the outer periphery of the cladding 20 may be the same as that of the outer periphery of the cladding 20 in the first embodiment. In this case, only one of the portions of the outer periphery of the cladding 20 facing the cores 10 at both ends is designated as the first outer periphery 21, and the other portions of the outer periphery of the cladding 20 are designated as the second outer periphery 22.

[0046] Although the present invention has been described above with reference to the above embodiments, the present invention is not limited thereto. For example, the first outer peripheral surface 21 may be provided on the outer peripheral surface of the cladding 20 in a portion other than the portion facing the core 10.

[0047] Furthermore, the first outer peripheral surface 21 may be provided on some of the multiple portions of the outer peripheral surface of the cladding 20 that face the core 10. The multicore fiber 1 of the first embodiment is an example of this configuration. In addition, in the first embodiment, the first outer peripheral surface 21 may be provided on two or three of the four portions of the outer peripheral surface of the cladding 20 that face the core 10. That is, some of the portions of the outer peripheral surface of the cladding 20 that face the core 10 and are located on the outer peripheral side may be designated as the first outer peripheral surface 21, and other portions of those portions may be designated as the second outer peripheral surface.

[0048] Furthermore, in the first and second embodiments, all cores 10 were located on a circumference centered on the reference position 20R, but other cores may be arranged inside the circumference. For example, a core may be arranged on the reference position 20R.

[0049] Furthermore, in the above embodiment, an example was described in which the length of the straight line connecting both ends of the first outer surface 21 is equal to or greater than the diameter of the core 10 facing the first outer surface 21. However, the length of the straight line connecting both ends of the first outer surface 21 may be smaller than the diameter of the core 10. However, as in the above embodiment, if the length of the straight line connecting both ends of the first outer surface 21 is equal to or greater than the diameter of the core 10, higher-order modes of light that are unnecessary for communication can be absorbed more efficiently.

[0050] Furthermore, in the above embodiment, when viewing the first outer surface 21 along a direction perpendicular to the longitudinal direction of the multicore fiber 1, the core 10 facing the first outer surface 21 completely overlaps the first outer surface 21. However, when viewed in the same manner, a portion of the core 10 facing the first outer surface 21 may overlap the first outer surface 21, while other portions may not overlap the first outer surface 21. However, if the core 10 facing the first outer surface 21 completely overlaps the first outer surface 21, it is possible to efficiently absorb higher-order modes of light that are unnecessary for communication.

[0051] As described above, the present invention provides a multicore fiber that can be easily aligned and can be used in the field of optical communication and other devices that utilize multicore fibers.

Claims

1. A structure comprising four cores and a cladding surrounding the four cores, The outer circumferential surface of the cladding has a first outer circumferential surface which is a part in the circumferential direction and a second outer circumferential surface which is another part in the circumferential direction. The first outer surface bulges outward from the cladding with a larger radius of curvature than the second outer surface. The four cores are arranged on the outer circumference of the cladding in a rotationally symmetrical manner four times. Each of the portions of the outer circumferential surface facing the four cores is the first outer circumferential surface. A multicore fiber characterized by the following features.

2. The radius of curvature of the first outer surface is 1.5 times or more and 20 times or less the radius of curvature of the second outer surface. The multicore fiber according to feature 1.

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